Stir and turn the beef strips, breaking apart any that are stuck together. Continue cooking for another 5 to 7 minutes, stirring occasionally, until most of the liquid has cooked off and the meat is no longer visibly pink. If the pan gets too dry or starts to scorch, turn the heat down slightly.
Add the sliced onion to the skillet with the beef. Cook, stirring often, for 4 to 5 minutes, until the onion softens and starts to turn golden around the edges. Scrape up any browned bits from the bottom of the pan as the onion releases its juices.
Stir in the drained canned mushrooms. Season the mixture with salt, black pepper, and garlic powder if using. Cook for another 2 to 3 minutes so the mushrooms can heat through and pick up some of the flavor from the beef and onions.
Turn the heat down to medium-low. Pour in the heavy cream, stirring as you go, and scrape the bottom of the skillet to loosen any remaining browned bits. Let the mixture simmer gently for 3 to 5 minutes, stirring now and then, until the cream thickens into a silky gravy that clings to the beef and vegetables.
Taste and adjust the seasoning with more salt and pepper if needed. If you like a touch of herb flavor, sprinkle in the dried thyme or parsley and stir it through just before serving.
Serve the creamy beef strips hot, spooned over your favorite low carb base such as steamed green beans or cauliflower rice, making sure everyone gets plenty of that comforting skillet gravy.
